As treatment using balloon dilation catheters, for example, Percutaneous Transluminal Angioplasty Catheters (i.e., “PTACs”), has progressed into narrower and more remote vessels within the body, this has necessitated the use of catheters having correspondingly smaller shaft diameters and longer shaft lengths. However, this migration towards catheters with smaller diameter, longer shafts has produced several new problems of its own. First, catheter inflation/deflation time performance (i.e., the time required for inflation and deflation of the balloon) has suffered (i.e., increased) in the longer, smaller diameter catheters as frictional resistance to movement of the inflation fluid through the balloon inflation/deflation lumen(s) of the catheter becomes significant. Second, accessing increasingly smaller body lumens requires that the distal portion of the catheter shaft have sufficient longitudinal flexibility to follow the guidewire as it twists and turns through the smaller, more arduous pathways in the body. If the distal end of the catheter is not flexible enough, it may pull the guidewire out of position in the target lumen rather than follow it. The flexibility of the distal portion of the catheter is often referred to as the “trackability” of the catheter.
Further, the operating pressure of dilation balloons continues to rise. Whereas at one time a balloon inflation pressure of 10 atmospheres was considered high, now balloons having operating pressures of up to 30 atmospheres are known, and it is foreseeable that even higher pressures may be utilized in the near future. It will be appreciated that use of such high pressure balloons requires the use of catheters having correspondingly high pressure ratings for their inflation/deflation lumens.
One conventional approach to improving inflation /deflation time in PTAC and other balloon catheter shafts is the so-called coaxial design, wherein two concentrically disposed tubular members form the catheter shaft. In coaxial catheters, the inside of the inner tubular member is used for the guidewire lumen, the outer tubular member is used for the catheter shaft body, and the annular space between the outer surface of the inner tubular member and the inner surface of the outer tubular member forms a inflation/deflation lumen for transporting contrast media or other non-compressible fluid for inflating and deflating the balloon. In such coaxial catheters, the inflation/deflation performance is governed by the difference in cross-sectional area between the inside diameter of the outer tubular member and the outside diameter of the inner tubular member in conjunction with the length of the shaft. For a given combination of overall catheter diameter and guidewire lumen diameter, the coaxial design is considered to maximize the cross-sectional area available for the inflation/deflation lumen, thereby providing the best inflation/deflation performance for a given length catheter.
Unfortunately, while the inflation/deflation time performance of coaxial designs is considered good, the trackability of such designs tends to be poor because of the inherent stiffness of the full-diameter outer tubular member. It will be appreciated that, generally speaking, a larger diameter tube will exhibit greater stiffness than a smaller diameter tube made of the same material. Further, as balloon working pressures increase, the outer tubular member must have thicker and/or stronger walls to withstand the pressure. Thus, as the pressure rating of a coaxial style catheter increases to handle higher-pressure balloons, its trackability characteristics tend to become even worse.
In an effort to provide better trackability, it is known to produce balloon catheter shafts wherein the guidewire lumen and the inflation/deflation lumens are separate and non-coaxial. Such non-coaxial designs are often referred-to in the industry as “multi-lumen” catheters, even though it is appreciated that the coaxial designs effectively have multiple lumens as well. In keeping with industry practice, for purposes of this application, the term “multi-lumen” will refer to designs wherein the guidewire lumen and inflation/deflation lumen(s) are not coaxial. There are currently two main types of multi-lumen catheter shafts: dual lumen shafts and extruded dual port shafts. In dual lumen shafts, a first tubular member forming the guidewire lumen therewithin and a second tubular member forming the inflation/deflation lumen therewithin run parallel to one another within a full-diameter outer jacket surrounding both lumens. Since only the guidewire lumen member and the inflation/deflation lumen member (i.e., not the outer jacket) are exposed to the balloon inflation pressure, then only these relatively small diameter tubular members need to be strong enough withstand such pressures, and the full-diameter outer jacket of the catheter can be made of a relatively soft, flexible material. This combination of smaller diameter (and hence more flexible) high-pressure tubular lumen members and a softer full-diameter casing gives dual lumen catheter shafts significantly more flexibility and trackability than coaxial designs.
The other type of multi-lumen catheter shaft, i.e., the extruded dual port shaft, has guidewire and inflation/deflation lumens that are integrally formed longitudinal voids created during extrusion of the plastic or resin catheter shaft. These extruded shafts tend to have good trackability, especially when extruded from relatively soft plastic/resin, due to their lack of any internal structural stiffeners. Further, their extruded construction allows the inflation/deflation lumen to be non-circular in shape, e.g., semi-circular or even crescent-shaped, to increase the portion of the catheter's total cross-sectional area that is available for the inflation/deflation lumen. However, unless the lumen members are lined with a stronger material, the maximum balloon inflation pressure rating of extruded dual port shafts tends to be low compared to other designs. Increasing the strength of the plastic/resin used for the extruded body will improve the pressure rating, but only at the expense of trackability. For this reason, the use of extruded dual port catheters becomes more problematic as the working pressure of balloons rises.
Regrettably, while multi-lumen catheter shafts exhibit improved trackability over the coaxial designs previously described, both types of multi-lumen catheter shafts have inflation/deflation time performance that is inferior to the coaxial designs. The reduced-size inflation/deflation lumens of the multi-lumen designs simply do not have fluid flow characteristics that are as good as the annular inflation/deflation lumen of the coaxial designs. Accordingly, practitioners are currently forced to choose between balloon catheters, which have good inflation/deflation performance but inferior trackability, good trackability but inferior inflation/deflation time performance, or a less-than-optimum compromise on both characteristics.
A need therefore exists, for a balloon catheter with a shaft having inflation/deflation time performance similar to that of a conventional coaxial design, but with improved trackability in the distal portion.